1. Field of the Invention
The present invention relates to a method of appraising a dielectric film, a method of calibrating the temperature of a heat treatment device, and a method of fabricating a semiconductor memory device; and in particular, to an ideal method of appraising a dielectric film, this appraisal in turn being used to appraise the crystallinity of a dielectric film that is grown on a DRAM capacitor; a method of calibrating the temperature of a heat treatment device; and a method of fabricating a semiconductor memory device.
2. Description of the Related Art
In recent years, the development of large-capacity memory elements has been advancing with the development of semiconductor fabrication technology. The progress toward higher integration of DRAM (Dynamic Random Access Memory) in which one memory cell is constituted by a single capacitor and a single transistor has been particularly rapid. Although capacitors of higher capacitance are required in order to maintain reliability such as resistance to soft errors in semiconductor memory devices such as DRAM, the area that can be occupied by capacitors is decreasing as semiconductor memory device become more highly integrated.
Since the charge storage capacity of a capacitor increases in proportion to the electrode area of the capacitor and the dielectric constant of the dielectric film and in reverse proportion to the thickness of the dielectric film, various methods have been proposed for increasing the electrode area of capacitors. Known methods include methods in which the (storage node) electrodes themselves are processed to a fin shape or crown shape, or methods in which HSG (Hemi-Spherical Grains) are formed.
As a simplified explanation of this HSG formation mechanism, when an amorphous silicon film having a clean surface is heated to a temperature sufficient to cause crystallization, the silicon atoms are dispersed within the film with high mobility, and collisions between these silicon atoms result in the formation of crystal nuclei. Crystallization centering on these crystal nuclei progresses from the surface of the film in the direction of depth of the film, and hemispherical crystals having a diameter of several tens of nanometers are formed, thereby producing a minute roughness is produced on the surface.
The use of this HSG technique enables the formation of capacitors having more than twice the surface area of a flat surface.
Alternatively, as methods of increasing the capacitance of a capacitor without changing the electrode area, research is in progress both regarding the reduction of the thickness of the dielectric film and regarding dielectric films that have a high relative dielectric constant. In recent years, tantalum pentoxide (Ta2O5), yttrium oxide (Y2O3), and hafnium dioxide (HfO2) are receiving attention as films having a high dielectric constant. These materials have a relative dielectric constant that is markedly higher than a silicon oxide film, which has a relative dielectric constant of 3-4, or a silicon nitride film, which has a relative dielectric constant of 6-8. Tantalum pentoxide, which is a material having inherent-thermodynamic stability, is particularly promising as a material for DRAM capacitors.
Tantalum pentoxide has a relative dielectric constant on the order of 22-25 even as a thin film, and a film can therefore be grown by sputtering, CVD (Chemical Vapor Deposition), or sol-gel. After formation of the tantalum pentoxide film, the tantalum pentoxide is next subjected to a heat treatment in an oxygen atmosphere to raise the relative dielectric constant to approximately 40. The chief objective of this heat treatment is to improve the crystallinity of the tantalum pentoxide film, and the treatment is carried out in an oxygen atmosphere to maintain the crystallinity of the tantalum pentoxide film.
Thus, an increase in the capacitance of the capacitors is achieved by: forming HSG on the capacitors of a semiconductor memory device such as DRAM to increase the surface area, growing a film of a material having a high dielectric constant such as tantalum pentoxide on this roughened surface, and then subjecting the surface to a heat treatment. In a DRAM fabrication method that employs this tantalum pentoxide film, however, there is the problem that the effect of the heat treatment on the tantalum pentoxide film cannot be adequately judged.
If the area of the electrode is fixed as described above, the electrostatic capacity of the capacitors varies depending on the relative dielectric constant and film thickness of the-dielectric film. Although reproducibility of the film thickness of the dielectric film can be secured by adjusting the conditions of the CVD film-forming device, the dielectric constant of the dielectric film varies depending on the crystallinity of the dielectric film itself, and therefore varies greatly according to the temperature condition of the heat treatment.
This problem is further explained with reference to FIG. 3, which shows the correlation between the temperature (RTO temperature) of the heat treatment and the refractive index of the dielectric film. As shown in FIG. 3A, the refractive index, which indicates the crystallinity of the dielectric film, does not gradually increase with increase in the temperature of the heat treatment, but rather, changes abruptly with a particular temperature as a line of demarcation and then increases to a fixed value. This phenomenon occurs due to a rapid improvement in the crystallinity of the tantalum pentoxide from an amorphous state to a crystalline state when the heat energy exceeds a particular threshold value, and after this improvement in crystallinity, density does not change despite further increase in temperature.
Accordingly, to improve the crystallinity of a dielectric film and set the dielectric constant to a desired value, the temperature of the heat treatment is preferably set as high as possible. However, components such as transistors are formed in layers below the capacitors of a semiconductor memory device, and setting the temperature of the heat treatment to a high level results in problems such as change in the distribution of impurity concentration in diffusion layers and the alteration of the characteristics of transistors due to the diffusion of impurities into unintended areas. Thus, when actually fabricating a semiconductor memory device such as DRAM, the temperature of the heat treatment is preferably set to the vicinity of the border between area II and area II in FIG. 3A. Since this is an area in which the refractive index changes rapidly with respect to change in temperature, the temperature of the heat treatment must be: set accurately.
In semiconductor fabrication devices such as heat treatment devices, the treatment temperature is typically controlled based on the indication of a temperature sensor that is installed inside the device, but factors such as the position of installation of the temperature sensor inside the device or the shape and amount of the sample that is being used may result in divergence between the temperature that is indicated on the device and the actual treatment temperature. This divergence may further vary over time according to factors such as the operation time of the device.
In order to correct this divergence in temperature, a method may be adopted in which a sample for temperature calibration on which a dielectric film has been grown is used to actually carry out the heat treatment, following which the crystallinity of the dielectric film after the heat treatment is then appraised by, for example, an x-ray diffraction method to estimate the actual treatment temperature and adjust the set temperature of the device. However, this approach is problematic both because the x-ray diffraction method takes time to perform and because a correspondence cannot be established between the x-ray diffraction data and the treatment temperature after the crystallinity of the dielectric film has been improved, whereby calibration data cannot be obtained for the vicinity of the point of inflection that is of utmost important in the heat treatment device.
In addition, when using the above-described x-ray diffraction method to appraise a dielectric film in an actual fabrication process of DRAM, a wafer product cannot be used in appraisal because x-rays are emitted in the x-ray diffraction method, and further, the appraisal depends on the underlying constituent substances. It is therefore necessary to place a dummy wafer for measurement purposes inside the film-forming device when growing the dielectric film and subject the dummy wafer to an oxidation treatment at the same time as the fabricated product to produce a sample for measurement purposes, and this requirement entails extra production steps. Even so, the dummy wafer will not necessarily be identical to the actual product due to differences in pattern and the state of the rear surface.
Furthermore, although the crystallinity of the dielectric film that is formed on the uppermost layer of the sample for measurement can be determined by the above-described x-ray diffraction method, the capacitance of the capacitors in an actual DRAM is reflected by the dielectric constant of not only the dielectric film of the uppermost layer, but of the entirety of laminated films that include the silicon oxide film, silicon nitride film, and polysilicon that are formed below the uppermost layer, and appraisal by x-ray diffraction therefore does not appraise the capacitance of the capacitors. In particular, oxygen atoms penetrate the dielectric film during the oxidation treatment and reach the silicon wafer, where a silicon oxide film forms on the silicon wafer interface; and the capacitance of the DRAM as a fabricated product cannot be accurately gauged if the dielectric constant of this entirety of laminated films is not measured.
Thus, there is the problem that, although an improvement in the surface area of the capacitance electrode, an improvement in the relative dielectric constant, and an increase in capacitance can be contrived by adopting an HSG construction for capacitors and by using a tantalum pentoxide film for the dielectric film, the lack of an effective means of measuring the total relative dielectric constant of the dielectric films in the capacitor means that the electrostatic capacitance of capacitors cannot be obtained with accuracy until an actual DRAM has been completed.
The present invention was achieved in view of the above-described problems and has as a first object the provision of a method of appraising dielectric films that enables easy and reliable estimation of the crystallinity or relative dielectric constant of a dielectric film, and in particular, laminated dielectric films without need for providing a dummy wafer.
It is a second object of the present invention to provide a method-of calibrating the temperature of a heat treatment device that enables accurate correction of: divergence between a set temperature and the actual treatment temperature in a heat treatment device; divergence between the treatment temperatures of different devices; and fluctuation over time in the treatment temperature of individual devices.
It is a third object of the present invention to provide a method of fabricating a semiconductor memory device that, following heat treatment of a dielectric film, enables accurate estimation of the performance of DRAM capacitors as a fabricated product.
To achieve the above-described objects, the method of appraising dielectric films in the present invention is a method of appraising dielectric films that are deposited on a substrate; wherein at least one of change in the crystallinity and change in the relative dielectric constant of the dielectric film before and after a heat treatment that is performed in an atmosphere that contains oxygen is appraised by measuring the refractive index of the dielectric films.
In the present invention, the dielectric films can be composed of laminated films of a plurality of dielectric films each having different relative dielectric constants, and the invention can be constituted such that the refractive index of the laminated films is measured to estimate the refractive index of the entirety of the plurality of dielectric films.
The present invention may further be constituted such that the plurality of dielectric films includes: a dielectric film in which crystallinity is changed by the heat treatment, and a dielectric film in which film thickness is changed by the heat treatment.
In addition, the present invention may be constituted such that: the dielectric films preferably include any one of a tantalum pentoxide film, an yttrium oxide film, and a hafnium oxide film; the dielectric films are formed on silicon or a polysilicon film either in direct contact with the silicon or polysilicon film or with a silicon oxide film or silicon nitride film interposed, and a silicon oxide film is formed by the heat treatment at the interface of the silicon or the polysilicon film.
In the present invention, the refractive index is preferably measured by a spectral ellipsometer.
The present invention is a method of calibrating the temperature of a heat treatment device that subjects a dielectric film that has been deposited on a substrate to a heat treatment in an atmosphere that contains oxygen, wherein correlative data of the temperatures of the heat treatment and the refractive indices of the dielectric film that has undergone heat treatment at these temperatures are used to correct divergence between the set temperature of the heat treatment device and the actual treatment temperature.
The present invention may be constituted such that correlative data of the refractive indices of the dielectric films and the heat treatment temperatures that are obtained for each individual device of a plurality of the heat treatment devices are consulted to correct temperature differences between the plurality of heat treatment devices.
In addition, the present invention can be constituted such that correlative data of the temperatures of heat treatment and the refractive indices of dielectric films are obtained in advance for the heat treatment device, and the correlative data are then compared with data of the refractive index of a dielectric film that is subsequently subjected to treatment to correct for temperature fluctuation of the heat treatment device that occurs over time.
Still further, the present invention is a method of fabricating a semiconductor memory device that includes a step of carrying out a heat treatment in an atmosphere that contains oxygen after forming a dielectric film in capacitors; the step of subjecting the dielectric film to a heat treatment including using a portion of the substrate on which the semiconductor memory device is formed to measure, the refractive index of the dielectric film after the heat treatment to appraise at least one of: change in the crystallinity and change in the relative dielectric constant of the dielectric film, whereby the capacitance of the capacitors following completion of the semiconductor memory device is estimated.
In the present invention, a scribe line of the semiconductor memory device is preferably used for measuring the refractive index.
According to the constitution of the present invention, measurement of the refractive index of a dielectric film following an oxidation treatment enables measurement of, not only the crystallinity of the uppermost dielectric film, but the refractive index of the entirety of laminated films including a film that is formed at the interface, and thus enables an easy and reliable estimate of the capacitance of capacitors at a stage midway in fabrication. In addition, measurement in advance of correlative data of the refractive indices and heat treatment temperatures for each device enables detection of variations in the treatment temperatures between devices or of fluctuation in a device over time, and feedback of the results of comparing the correlative data with refractive indices that are measured after the heat treatment allows heat treatment devices to be kept in a uniform state. Furthermore, using a portion (such as a scribe line) of a semiconductor memory device such as DRAM to measure the refractive index enables measurement of the refractive index of a dielectric film that has been grown on an actual wafer product and allows accurate prediction of the cell capacitance of the semiconductor memory device.
The above and other objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings, which illustrate an example of a preferred embodiment of the present invention.